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The spatial constraints of electromagnetic design for mobile phones are more stringent than most consumer electronics. The NFC antenna, wireless charging coil, millimeter wave module, metal shielding cover, camera module, and battery shielding layer are compressed inside the body with a thickness of less than 8mm, and the distance between them is often only in the millimeter range.
At this density, magnetic separators and absorption plates are often arranged in the same area in front of the metal backplate. A straightforward idea is that since the magnetic separator is responsible for "magnetic conduction" and the absorbing plate is responsible for "absorption", stacking the two together and performing their respective functions should be able to simultaneously solve the problems of NFC reading distance and cavity resonance. But in actual testing, this kind of superposition often does not work.
Same metal backplate, impedance requirements in opposite directions
NFC and wireless charging work in the near-field magnetic field coupling region, where the distance between the antenna and the metal backplate is much smaller than the wavelength. The core function of the magnetic separator is to establish a high μ 'and low μ' magnetic flux bypass between the antenna and the metal. The meaning of 'low μ' is that the 'consumption' of magnetic flux by the material must be very small in order for energy to be effectively transmitted from the reader to the antenna, or from the coil to the receiving circuit. From the perspective of impedance, the side of the magnetic separator facing the antenna needs to present a boundary condition that is close to "magnetic flux can pass through", rather than a boundary condition where "energy is absorbed".
Millimeter wave absorbing materials face a different set of physical conditions. The cavity resonance and surface wave coupling in the 24-43.5 GHz frequency band are essentially standing waves formed by multiple reflections of electromagnetic waves between metal boundaries. The function of a waveplate is to reduce the Q value of the cavity through magnetic and dielectric losses, and dissipate the standing wave energy. This means that the absorbing layer needs to present a boundary condition of "energy entering and being consumed", and its input impedance should be as close as possible to the wave impedance of free space or target cavity, so that electromagnetic waves enter the material rather than being reflected on the surface.
The two have different requirements for the impedance on the side facing the antenna/cavity: the magnetic separator requires the magnetic flux to pass through with low loss, while the absorbing plate requires the electromagnetic wave to be lost after entering. When these two types of materials are stacked in the same space, the equivalent input impedance after stacking does not meet the requirements of NFC antennas for low magnetic loss loading, nor does it meet the requirements of millimeter wave cavities for broadband absorption.
The NFC side is the first to encounter problems: the μ "in the absorbing layer has become an additional load on the antenna
In terms of stacking sequence, if the absorbing layer is located between the magnetic separator and the NFC antenna, or if they are arranged adjacent to each other in the same plane, the magnetic loss component μ "of the absorbing layer will directly participate in the loading of the NFC antenna.
The magnetic field energy of NFC antennas at 13.56 MHz is inherently limited. When the magnetic flux emitted by the reader attenuates through space and reaches the mobile phone end, the energy available to excite antenna resonance is already very small. One of the design goals of the magnetic separator is to minimize the consumption of this energy by the material. If there is still a significant loss in the absorbing layer near 13.56 MHz, this loss will manifest as an increase in the equivalent series resistance of the antenna, a decrease in Q value, and a shortened read distance. Even more covertly, the dielectric constant of the absorbing layer also changes the capacitance distribution around the antenna, causing a shift in the resonant frequency.
Therefore, a material with good absorption performance at 24 GHz may experience NFC performance degradation before millimeter wave performance if its μ "at 13.56 MHz is not controlled and placed together with an NFC isolator.
The reverse problem on the millimeter wave side: the high μ 'of the magnetic separator has limited contribution to the cavity resonance, but may disturb the surface wave
Conversely, placing a high μ 'magnetic separator near a millimeter wave cavity or antenna can also cause problems.
High μ 'materials can effectively guide magnetic flux in the low frequency range (kHz to 13.56 MHz), but above 24 GHz, the natural and exchange resonance mechanisms of magnetic materials have changed, and the magnetization process relied on by high μ' no longer responds to high-frequency magnetic fields in the same way. Magnetic separators cannot provide effective broadband magnetic losses to lower the Q value of the cavity in the millimeter wave frequency band. Their own dielectric constant and geometric shape may also change the field distribution inside the cavity, causing the originally designed absorption layer working point to shift.
More specifically, when 5G millimeter waves propagate under the glass screen of a mobile phone, TM surface waves may be excited around 20 GHz, while TE modes propagate from around 30 GHz. To suppress such surface waves, it is necessary to design sub wavelength structures or thin absorbers with specific dielectric/magnetic loss spectra, rather than a high μ 'magnetic sheet optimized for 13.56 MHz flux guidance.
Decoupling Path 1: Frequency selective layering, allowing each layer to operate in its own frequency band
The simplest engineering decoupling approach is frequency selective layering: separating the magnetic isolation layer and the absorbing layer in the thickness direction, and adding an isolation layer with high impedance to the target frequency band between the two, so that low-frequency magnetic flux and high-frequency electromagnetic waves are "screened" before entering each other's functional layer.
This approach has been demonstrated in the research of NFC/WPT dual band antennas. Inserting a magnetic material with high permeability between the antenna and metal can improve both WPT transmission efficiency and NFC communication distance. However, the increase in antenna resistance during insertion can cause impedance shift, which needs to be suppressed in a targeted manner. In other words, the "effective loading" of magnetic sheets is itself an impedance balancing behavior, rather than simply putting high μ 'materials in to solve all problems.
For multi-layer structures where magnetic isolation and absorption coexist, the operational judgment is to confirm whether the insertion loss of the magnetic isolation layer in the millimeter wave frequency band is low enough (to avoid additional absorption of millimeter wave energy), and to confirm whether the μ "of the absorption layer near 13.56 MHz is suppressed to a negligible level. If suppliers can only provide the "best data" for their respective frequency bands and cannot provide the superimposed S parameters or impedance data, the actual effect of layered design cannot be predicted.
Decoupling path 2: Space partitioning, placing two types of requirements in different physical areas
When frequency selective layering is difficult to achieve within a limited thickness, spatial partitioning is a more realistic path: the magnetic separator only covers the projection area of the NFC/wireless charging coil, and the absorbing plate is only arranged around the millimeter wave module or the resonant sensitive area on the inner wall of the shielding cover. The two are staggered in the plane to avoid direct stacking in the thickness direction.
The engineering key of this path is the field distribution at the boundary. The magnetic flux at the edge of the magnetic separator will overflow outward. If the absorbing plate is adjacent to the edge of the magnetic separator, the overflowing magnetic flux may enter the absorbing layer and be partially consumed, resulting in additional losses in the NFC antenna edge area. The edge of the millimeter wave absorption layer may also cause impedance mutations in adjacent metal structures, exciting new surface waves. Space partitioning is not simply "cutting open", but requires leaving transition zones or impedance gradient structures at the boundaries.
Decoupling Path Three: Gradient Structure, Achieving "Low Frequency Low Loss, High Frequency High Loss" within the Same Material System
A further approach is to construct frequency dependent loss spectra within a single material system: maintaining low μ "in the low frequency range (13.56 MHz, 100-205 kHz), and exhibiting sufficient magnetic and dielectric losses in the high frequency range (24-43.5 GHz). The gradient permeability stack structure is an existing practice of this idea in NFC/WPT coexistence scenarios - the permeability increases by 8% -40% from the side closer to the NFC antenna to the side farther away, so that the low μ 'layer on the antenna side avoids loading detuning, and the high μ' layer on the far side undertakes magnetic flux guidance.
But to simultaneously cover millimeter wave absorption, the design space for gradient structures will be narrower. Millimeter wave absorption requires materials to have effective loss mechanisms at high frequencies (such as natural resonance, dielectric relaxation, or structural resonance), which are often constrained at the material level by the requirement for low loss at low frequencies. The current engineering reality is that it is difficult to perfectly meet both NFC low loss and millimeter wave broadband absorption with a single layer of material. Gradient stacking is more optimized within the scope of "magnetic isolation as the main factor, while also considering high-frequency suppression", rather than replacing dedicated millimeter wave absorption layers.
The following table summarizes the applicable boundaries of three decoupling paths:
Selection and judgment of electromagnetic materials for mobile phones based on impedance superposition
Firstly, before stacking, confirm the loss data of the two types of materials in the "opposite frequency band"
The supplier of the magnetic separator should provide the μ "at 13.56 MHz and the insertion loss or dielectric constant at 24-43.5 GHz; The absorber plate supplier shall provide their μ 'and μ' at 13.56 MHz and 100-205 kHz. If the absorption layer's μ "in the NFC frequency band is not controlled, the degradation of NFC reading range after superposition may occur earlier than the improvement of millimeter wave.
Secondly, distinguish between "optimal for each frequency band" and "usable after superposition"
The performance range of the magnetic separator at 13.56 MHz in the range of μ'300-500 and μ ″ 100-200 is a common range, but this set of data cannot be directly extended to the scenario of "coexisting with the absorbing layer". When selecting, suppliers should be required to provide measured changes in S parameters or NFC antenna Q values and resonant frequencies after stacking multiple layers of structures, rather than just providing specifications for single-layer materials.
Thirdly, prioritize using spatial partitioning to solve the problem of "opposite demand directions", and use gradient structures to solve the problem of "frequency selectivity required within the same area"
If NFC coils and millimeter wave modules can be separated in layout, spatial partitioning is a path with lower engineering risks, and the design of boundary transition zones is easier to verify than the inherent frequency selectivity of materials. If both must share the same projection area, it is necessary to confirm whether the gradient stack still has usable losses in the millimeter wave frequency band, rather than assuming that "high μ 'magnetic separators can also absorb millimeter waves".
Advanced Institute (Shenzhen) Technology Co., LtdThe product line of magnetic isolation and absorption materials for mobile phones covers ferrite magnetic isolation sheets, nanocrystalline magnetic isolation sheets, ultra-thin absorption plates, and fine pore absorption foam.
The magnetic separator series can provide frequency division data of μ 'and μ "at 13.56 MHz, supporting the design of layered composite schemes with absorbing layers; Fine pore absorbing foam is designed for cavity resonance and surface wave suppression at 24-43.5 GHz, with a thickness of 1-3 mm that can be customized.
The product supports rolls, sheets, and precision die-cutting, and can provide impedance matching evaluation of stacked structures based on the actual layout of NFC/wireless charging and millimeter wave modules. For more technical information, please visithttp://www.avanzado.cn/.